Added icon.

git-svn-id: https://plugins.svn.wordpress.org/wp-mail-logging/trunk@979665 b8457f37-d9ea-0310-8a92-e5e31aec5664
Assets are detected in sub directories except icons.

git-svn-id: https://plugins.svn.wordpress.org/wp-mail-logging/trunk@980007 b8457f37-d9ea-0310-8a92-e5e31aec5664